Chocolate

I've heard that it is dangerous to feed your dog chocolate, but can you please explain to me why? What is it in the chocolate, and why is it toxic to dogs, but not to humans (except in the case of atypical sensitivity or allergy)? Is it toxic to other animals? I must admit that my dog has had small amounts of chocolate, more or less accidentally (nothing like several ounces) with no apparent ill effects... Are some dogs/breeds more sensitive to it than others, as with most allergies in humans?

Here is the short story about chocolate. Chocolate is toxic to dogs, in
excess, because it contains theobromine. Theobromine is a
stimulant (like caffeine) which can cause irregular or excited
heartbeats. The stimulant affects the central nervous system, causing any
of the following signs: panting, diarrhea, restlessness, excitedness,
nervousness (though try to diagnose the previous three in any Border Collie! Hah!),
vomiting, depression, frequent urination, bluish mucous membranes, and as
it gets worse - muscle tremors, seizures, coma, and sudden death. This
can happen in as short a time as 6 hours or as much as 24 hours.

A 4 oz. chocolate bar can kill a 5 lb. dog.
It takes about 16 oz. to kill a 20 lb. dog.
There is no known treatment other than inducing vomiting or consumption
of activated charcoal.

If you're overly curious about this the following article is a good
resource: